Topography of locus ceruleus neurons projecting to the area dentata

The distribution of locus ceruleus neurons projecting to the dorsal and ventral area dentata was studied using the retrograde transport of horseradish peroxidase (HRP). Furthermore, the possibility that collaterals of single locus ceruleus neurons innervate both the dorsal and ventral area dentata was explored with a retrograde double-label technique using HRP and the fluorescent dye 4',6-diamidino-2-phenylindole 2 HCl (DAPI). Fusiform and multipolar neurons of the pars dorsalis of the locus ceruleus (LCd) were labeled after injection of HRP into the area dentata. The HRP-positive cells were found almost exclusively in the dorsal LCd after dorsal area dentata injections whereas ventral injections labeled neurons throughout the LCd. The highest density of LCd neurons labeled from the ventral area dentata was observed in the dorsal half of the LCd and thus overlapped with the distribution of neurons labeled from dorsal injections. Neurons containing both HRP and DAPI were observed in the dorsal LCd suggesting that the topographic coincidence of neurons labeled from dorsal and ventral area dentata was in part a result of single neurons having terminals in both loci.